It's my understanding that any sort of flashing brake light is illegal in most states. However, cops *love* my hyperlites. I have been complimented on them, by police officers, on a number of occasions.
Also, this past weekend I rode with a guy who had augmented his brakelight with an honest-to-god strobe light, which he ordered from a police catalog, and he reports having cops ask him about it, and tell him they liked it. And that is definitely an illegal light.
I think that, like earplugs, this falls in the category of "things which only get negative cop attention, when the cop is looking for an excuse to do what they've already decided to do to you." Cops don't have the law books memorized - they just do what they think makes sense. So you might as well make your decision based on safety, rather than on pleasing the police.